1. Field of the Invention
The present invention relates generally to a method and an apparatus for displaying halftone in a liquid crystal display. More particularly, the present invention relates to a method and an apparatus for displaying halftone in a liquid crystal display by applying two different voltages periodically.
2. Description of the Related Art
Typically, a liquid crystal display (LCD) device contains a display panel in which a plurality of pixel circuits including liquid crystal elements are configured in a matrix; an LCD driver for applying a predetermined data signal to the pixel circuits of the display panel; and a selecting driver for applying a predetermined scanning signal to the display panel to select a predetermined pixel circuit. FIG. 20 illustrates an equivalent circuit diagram of the pixel circuit included in the LCD device. As illustrated, this pixel circuit includes a data signal line 101 for applying a data signal from the LCD driver; a scanning signal line 100 for applying a scanning signal from the selecting driver; a field effect transistor 102 functioning as a switching device in response to the scanning signal; a liquid crystal element 103 for adjusting quantity of display light through the pixel; and an auxiliary capacitance element 104 having predetermined capacitance. The field effect transistor 102 is connected to the scanning signal line 100 at its gate, and to the data signal line at its drain. Also, one end of the liquid crystal element 103 and the auxiliary capacitance element 104 is each connected to a source of the field effect transistor 102, and another end of these elements is each connected to a common electrode Vcom which is a common electrode for all the pixel circuits. In this pixel circuit, when the scanning signal is provided, i.e., the scanning signal line 100 is selected, the field effect transistor 102 turns on, thereby causing a voltage (of the data signal) applied to the data signal line to be applied to the auxiliary capacitance element 104. Then, once the period during which the scanning signal line 100 is selected ends, the field effect transistor 102 turns off, while a voltage across the capacitance at the beginning of this off state is maintained by electric charge stored (or held) in the auxiliary capacitance element 104. Here, since light transmissivity or light reflectivity of the liquid crystal element 103 varies depending on the applied voltage, application of a voltage corresponding to image data to the data signal line 101 enables display luminance (or display tone) of the pixel to be varied according to the image data when the scanning signal line 100 is selected.
Here, a driver for driving a digital LCD device selects one of a plurality of predetermined reference voltages based on digital data given externally, and applies the selected reference voltage to a pixel element in a specific pixel circuit. As the number of gray levels increases, this digital LCD driver requires an increased number of elements included therein, and thus, ends up in higher manufacturing cost. To deal with this problem, frame rate control techniques (hereinafter referred to as the “FRC techniques”) have been developed as a control technique for increasing gray levels without increasing the number of devices used therein, while still using a digital LCD driver.
The FRC techniques achieve pseudo-halftone luminance for human eyes by applying two different driving voltages to a predetermined pixel element in multiple frames. FIGS. 21A-21C illustrate an example for multi-gray-level display (e.g., three-gray-level display) by an FRC technique. Here, two different driving voltages (i.e., “high” voltage and “low” voltage) are applied to a predetermined pixel element in adjoining frames 1 and 2, thereby allowing three-gray-level (i.e., multi-gray-level) display. In this case, the frame frequency is 60 Hz. An AC (alternate current) drive is performed by a frame inverting technique where the polarity of the applied voltage is inverted for each frame.
FIG. 21A illustrates a driving voltage in each frame for displaying a pixel A with lowest luminance, FIG. 21B illustrates a driving voltage in each frame for displaying a pixel B with highest luminance, and FIG. 21C illustrates a driving voltage in each frame for displaying a pixel C with a middle luminance. As shown in FIG. 21A, a low voltage with respect to the voltage potential VCOM of the common electrode applied in both frames 1 and 2 creates a pixel A with the lowest luminance. By contrast, as shown in FIG. 21B, a high voltage applied in both frames 1 and 2 creates a pixel B with the highest luminance. Then, as shown in FIG. 21C, a positive low voltage applied in frame 1, and a negative high voltage applied in frame 2 generate a pixel C with a middle luminance between the luminance of the pixel A shown in FIG. 21A and the luminance of the pixel B shown in FIG. 21B.
When the middle luminance shown in FIG. 21C is obtained by the FRC technique, a low-luminance pixel is displayed in frame 1, and a high-luminance pixel is displayed in frame 2. As a result, pixels with different luminances are displayed in adjoining frames. Thus, luminance variations of the displayed pixel C contains a flicker component which has a half of the frame frequency, i.e., 30 Hz. Here, in general, it is observed that flicker components whose frequency is lower than 50 Hz are noticeable for human eyes. Thus, if all pixels in a display screen are driven at the same phase as the middle-luminance pixel C shown in FIG. 21C, flicker components are conspicuous in the entire display screen. As a result, display quality of the display device would be deteriorated.
To address this issue, i.e., in order to achieve flickerless middle luminance of FIG. 21C, there have been techniques for spatially eliminating the associated flicker components by applying the driving voltage shown in FIG. 21C to some pixels, and by applying voltages different from that of FIG. 21C to other pixels, thereby allowing pixels displayed by these pixel circuits to be evenly positioned within a display screen in a diffused manner.
For example, in order to apply voltages different from that of FIG. 21C, the following three patterns may be used. In a first pattern, a positive high voltage is applied in frame 1, and a negative low voltage is applied in frame 2. In a second pattern, a negative high voltage is applied in frame 1, and a positive low voltage is applied in frame 2. In the third pattern, a negative low voltage is applied in frame 1, and a positive high voltage is applied in frame 2. FIGS. 22A-22C illustrate the above patterns in which these voltages are applied. FIG. 22A shows a driving voltage for the first pattern, and a pixel D generated by applying the driving voltage for the first pattern. FIG. 22B shows a driving voltage for the second pattern, and a pixel E generated by applying the driving voltage for the second pattern. FIG. 22C shows a driving voltage for the third pattern, and a pixel F generated by applying the driving voltage for the third pattern. These pixels D, E and F; and the pixel C shown in FIG. 21C are positioned to be diffused spatially, i.e., their positions in the display screen are evenly allocated. FIG. 23 illustrates an exemplary allocation of the pixels C, D, E, and F. FIG. 23 illustrates pixels in four rows for each of columns 1-4 where symbols C, D, E, and F in the figure correspond to the above-identified pixels C, D, E, and F, respectively. Positioning each pixel in this manner allows pixels generated by application of the same driving voltage to be diffused in the display screen in a specific frame, thereby spatially eliminating the above-mentioned flicker components.
According to the conventional FRC techniques described above assume that luminances of pixels generated by applying the same combination of driving voltages, e.g., luminances of pixels C, D, E, and F are identical with each other. However, there exists parasitic capacitance in the equivalent circuit including the field effect transistor 102 shown in FIG. 20. As a result, when two cases in which positive and negative voltages having the same driving voltage (high voltage or low voltage) are applied are compared with each other, the voltage potentials VDL, VDH, and VDM1-VDM4 shown in FIGS. 21A-21C and 22A-22C can be shifted from ideal levels. Thus, pixels generated by applying the same combination of driving voltages may have different luminances. For example, the middle luminance of pixels C and E, and the middle luminance of pixels D and F may be different from each other although these middle luminances should be the same.
The above-mentioned luminance differences may be noticeable for human eyes especially when flicker components are to be eliminated spatially. That is, the pixels C and E in the columns 1 and 3 are generated by being driven only by high voltages with positive polarity and negative polarity as shown in FIGS. 21C and 22B. The pixels D and F in the columns 2 and 4 are generated by being driven only by high voltage with positive polarity and low voltage with negative polarity as shown in FIGS. 22A and 22C. As a result, the luminances are the same on every other column while adjoining columns have different luminances with each other. Thus, stripe-shaped variations in luminance extending along the column direction is conspicuous for human eyes. Even if the pixels C and F shown in FIG. 23 are interchanged, this problem still remains, and in such a case, stripe-shaped variations in luminance extending along the row direction would be conspicuous.